1. Field of the Invention
The present invention provides an imaging system and method for selectively attenuating narrow band bright sources which cause a focal plane array to saturate, in which as a result, image contrast is lost from the incoming light source. In particular, the present invention utilizes a tiltable line attenuation tunable optical filter (LATOF) within imaging systems to shift the center of the wavelength rejection band along the wavelength domain, if required, for continuously tuning and/or selectively attenuating the narrow band bright source, as required, so that contrast can be restored.
2. Background of the Invention
In an electronic imaging system, especially one with high optical gain, there is an inherent sensitivity to bright sources. These sources cause the focal plane array (FPA) to saturate, and as a result, image contrast is lost in the area of the source. In some cases this bright source is broadband; in others the bright source is narrowband. In an effort to restore contrast, various attempts have been made to attenuate the bright source, which are now herein briefly discussed.
The most common manner in which bright sources are attenuated in imaging systems is to utilize an automatic gain control (AGC) loop. The AGC loop monitors the electronic output from an opto-electronic detector and sets the gain of that signal to best display the scene with an optimal contrast. In the event that a bright source enters the frame, the AGC loop will reduce the gain of the signal in an attempt to get the pixels out of saturation to improve contrast. This reduction of the gain will reduce the contrast of the overall scene, even if the source only occupies a small fraction of the scene. In this situation, prior art typically “flips-in” a spectral filter that blocks the bright source spectrally, and allows the AGC to re-adjust the gain for better contrast. The short-fall with this approach is that the filter is broadband, blocking up to 70% of the incoming light signal as well providing compromised contrast, even at maximum gain.
Other approaches have been attempted to attenuate the bright source independent of AGC loops. For instance, there are a variety of filters which can be used to selectively attenuate the bright source. Attenuation filters are used to reduce the intensity of a light beam. High quality attenuation filters are said to have a “flat response”, meaning that they attenuate all wavelengths of light over their usable spectral range by the same amount. Attenuation filters are used over a photosensitive surface when the light signal received is too intense. An example of this application is a light signal being measured by a photo detector. If the photo detector is responding linearly, the insertion of a 50% attenuation filter in the light beam should cause a 50% reduction in the output electrical signal.
There are two basic classes of attenuation filters, including geometric filters and neutral-density filters. Geometric filters, such as iris diaphragms or screen or mesh filters, physically block a fraction of an aperture through which the light beam passes. Neutral-density (ND) filters are uniform, “grey” filters that absorb and/or reflect a fraction of the energy incident upon them. The term “neutral” is designated because the absorption and/or reflection characteristics of the filter are constant over a wide wavelength range. Several types of neutral-density filters are available, including plastic or gelatin filters, absorbing glass, and metallic film on glass.
Another variant of a ND filter is a filter wheel which is adapted to continuously vary the intensity of the light beam. A filter wheel typically has a number of specific band rejection filters that are selected to block the incident light source. The number of specific filters must be a balance between the width of the rejection band and the size of the filter wheel that the package can accommodate. Typically, the number of filters is kept to two or three so the wheel size can be kept small, which means that the rejection bands must be wide to cover all wavelengths. This wide rejection band will attenuate up to half of the light the system uses to see, and thus, decrease functionality of the system overall while contrast may still be compromised due to low light.
Another classification of filters are wavelength-selective filters. Wavelength-selective filters are used to produce or select specific color or a band of color from a white light source, to isolate a specific wavelength, or to reject a specific wavelength or band of wavelengths. There are three general classes of wavelength-selective filters, including cut-off filters, bandpass filters, compensating filters, and notch or minus filters.
Cut-off filters have an abrupt division between regions of high and low transmission. If a filter transmits the shorter wavelengths, it is called a short-wave-pass filter or a low-frequency-pass filter. If a filter transmits the longer wavelengths and rejects the shorter wavelengths, it is called a long-wave-pass-filter or high-frequency-pass filter. Bandpass filters can be produced that transmit only a very narrow wavelength range. For instance, one important application of such filters in electro-optics is the isolation of individual laser lines. Compensating filters are designed to have gradually sloping spectral curves.
Other types of filters, such as narrow bandpass filter (e.g., crystal tunable filters), have been implemented for tunable band rejection as well. These filters work via polarization rather than interference and have properties that are easier to vary than a typical interference filter. For instance, the liquid crystal sits between two linear polarizers and can be electrically tuned to rotate the polarization of a specific narrow band of frequencies to cause the second linear polarizer to block those wavelengths. Currently, these types of filters can operate from the visible through short wave infrared bands.
Finally, interference filters may also be used for wavelength selection when a sharp cut-off or very narrow bandpass is required. Interference filters are generally made by depositing multiple alternating layers (thin coatings) of dielectric materials on a dielectric substrate or transparent substrate, such a glass or quartz window. Selection of materials and thickness of the coating are chosen to provide reflection or transmission at the desired wavelengths. When the number of layers is increased, the cut-off or the passband typically becomes sharper. An interference filter may have as many as 100 layers of coatings. These multilayer coating techniques are the same as are used to produce high-reflective laser mirrors. Absorption and scattering from these surfaces is typically less than 1%. Consequently, when these coatings are used as filters, practically all of the light is either transmitted or reflected. The spectral transmittance performs the filtering action. Also, since the transmitted beam shows negligible distortion from the interference coating, these filters can be used in imaging systems.
It would be beneficial to utilize one or a combination of the aforementioned filters to selectively attenuate unwanted bright sources in electronic imaging systems, especially with imaging systems which have high optical gain. Some of the primary design parameters that must be considered in providing a solution to help restore contrast include the attenuation and/or filtering characteristics of the specific filter design, hardware implementation, and ability to automate, to name a few. Ideally, it would be advantageous to provide a system which is capable of selectively attenuating both the broadband and narrowband bright sources. However, with the current state of technology, such a solution could prove too complex and/or costly to be feasible. In the alternative, is would be quite beneficial and useful to provide a solution which is useful for selectively attenuating the narrowband sources so that contrast can be restored to imaging systems in a manner better than the prior art systems provide.